Your chest starts moving, then your hips sink toward the floor. That is the moment your push up stops being one strong movement and turns into two separate ones.
The fix is simple to understand. Your chest and hips should rise and lower together.
Why your hips drop
A push up is not only a chest exercise. Your trunk has to hold a firm line from your shoulders through your hips while your arms move you up and down.
When your abs and glutes lose tension, gravity pulls the pelvis down. You might still finish the rep, but you are no longer controlling the same body position.
Fix it with three cues
1. Brace your abs before you move
Set your hands, straighten your legs, and gently tighten your stomach as if someone is about to tap it. Do this before the first rep, not halfway through it.
2. Squeeze your glutes
Your glutes help keep your pelvis from tipping forward. Use enough tension to keep the hips level without turning the position into a rigid, uncomfortable pose.
3. Move your chest and hips together
Lower your whole body as one unit. On the way up, imagine the floor pushing your chest and hips away at the same speed.
Record one rep from the side. If the distance between your hips and the floor changes before your chest moves, reset and try again.
If you still lose the line
Make the movement easier while keeping the same shape. Put your hands on a sturdy bench, rail, or box and perform an incline push up. Raise the surface until you can keep your chest, ribs, hips, and legs moving together.
That is a better rep than forcing a floor push up with a position you cannot yet control.
Your next rep
Before you lower, think: brace, squeeze, move together.
Three quick cues. One stronger push up.